Alternating current (AC) electric motors rely on alternating currents passed through induction windings within the stator to cause rotation of the rotor. So-called three phase AC motors include three matched sets of windings positioned radially about the stator. By supplying sinusoidal AC power to each of the sets of windings such that each set receives an alternating current offset by 120 degrees, a largely continuous torque can be imparted on the rotor as it rotates.
Unlike a brushed DC motor, output speed in an AC motor is controlled by the frequency of the current sent to the stator windings. In order to control output torque, and thus speed, a variable frequency drive (VFD) may be used to vary the current fed to the AC motor. Because the inductive reactance of the stator windings is proportional to the frequency applied to the windings, increased voltage is necessary to maintain a relatively constant current within the windings, and thus a relatively constant output torque up to the motor's rated speed. The rated speed generally corresponds with the rated voltage (usually equal to the supply voltage of the VFD). Past the rated speed, the speed of the AC motor may be increased through field weakening as discussed hereinbelow. This control algorithm is typically referred to as volts/Hz or V/Hz control.
In some electric motors, both the rotor and stator include coils. In such an induction motor, the magnetic field induced by the stator coils induces current within the rotor coils which, due to Lenz's law, causes a resultant torque on the rotor, thus causing rotation.
In a permanent magnet motor, on the other hand, the rotor includes one or more permanent magnets. The permanent magnets, in attempting to align with the magnetic field induced by the coils in the stator, cause a resultant torque on the rotor. By varying the orientation of the magnetic field, the rotor may thus be caused to rotate. In high-torque permanent magnet motors, such as for an internal rotor permanent magnet motor, multiple permanent magnets may be positioned on the exterior of the rotor.
VFDs typically use one of two control methods. In a Volts/Hz control scheme, the VFD varies the output speed of the motor by supplying AC power to the stator windings at a particular frequency and voltage. For a given desired torque, voltage is proportionally related to the frequency by a so-called “voltage-to-frequency” or “volts/Hz” ratio. As understood in the art, the impedance of an electric motor includes a static impedance and a back-EMF or counter-EMF. The static impedance is determined by the winding arrangement and construction of the motor. The counter-EMF is created by, in a permanent magnet motor, the current induced in the motor windings by the rotating magnetic fields of the permanent magnets. Accordingly, counter-EMF is negligible when the motor is at a standstill. As the speed of the motor increases, the counter-EMF likewise increases requiring additional drive voltage to be applied to the motor in order to maintain sufficient current through the windings of the motor. Traditionally, the drive voltage is supplied by the volts/Hz ratio, which is typically a constant through the normal operating range. Throughout this normal operation range, current is maintained in phase with the rotor. At a certain speed, the counter-EMF voltage reaches or exceeds the output voltage capability of the motor driver, at which point the phase of the current supplied to the motor is modified in order to, in a permanent magnet motor, inject negative polarity flux into the permanent magnets, effectively reducing their magnetic fields and thus the counter-EMF. By using closed-loop feedback, a VFD using volts/Hz can maintain motor speed in changing conditions. This simple form of volts/Hz may not allow accurate torque control.
With the rapid advancement in low-cost, high speed microprocessor technology, VFDs utilizing so-called vector control or field-oriented control (FOC) models have been used. In FOC, the current supplied to the phases of the AC motor is decoupled into torque and flux components acting on the rotor in a rotating reference frame. Thus, each of these components may be independently controlled. Current supplied to the phases of the motor are measured or derived and transformed into the torque-flux space (utilizing, for example, a Clarke/Park transformation), a closed-loop feedback model can be created to control each of these components continuously. The processor then back-transforms the torque and flux components into three phase currents. The three phase currents are fed to a three phase inverter which outputs pulse-width modulated signals to each set of windings in the motor.
In any AC motor, even under FOC, as the speed of the permanent magnet motor is increased, the voltage generated by the fixed magnetic field (EMF) increases proportionally. At a certain speed, the voltage generated by the motor exceeds the maximum voltage that can be produced by the drive that is controlling the motor. If operation above this speed is desired, it is necessary to modify the current vector applied to the motor to maintain the desired torque, and control the terminal voltage of the motor to a value less than the maximum drive output voltage. Control in this speed range is known as “field weakening control.” As the speed of the motor increases, the torque available decreases when in field weakening. At some speed, the available torque will not be enough to sustain operation of the motor. The ratio between this maximum speed and the rated or base speed of the motor is known as a speed ratio. Typically a permanent magnet motor may achieve a 3:1 speed ratio, an induction motor may achieve a 5:1 ratio, but typical electric motors may achieve between a 2:1 and 3:1 speed ratio.